1. Field of the Invention
This invention relates to a method and apparatus for processing a photosensitive material and specifically for washing photographic material.
2. Description of the Related Art
Processing of silver halide films as used in this application comprises subjecting silver halide film sheets or webs to development, fixing and washing stages. The film typically, but not necessarily, carries a latent image thereon which is rendered visible and permanent as a result thereof.
Equipment to facilitate and speed the processing is widely available in the art. Such equipment typically comprises a series of tanks or similar stations through which the film is passed sequentially. Each station contains either a developer chemical, a fixer chemical or water for washing the chemicals off the surfaces of the film once the desired chemical's effect on the film has been achieved.
The developer or fixer chemicals can be used in small quantities, in concentrated form and appropriately replenished, thus permitting their handling without the need for substantial equipment, if any, external to the apparatus plumbing. The wash stages on the other hand, heretofore have typically required a substantial amount of fresh water flow and associated plumbing to supply the fresh water as well as to provide an outlet for the wash water after it has been used to wash the film.
In order to minimize the amount of fresh water used in the wash stages of a processor, it is known to use multiple wash stages employing either a counter current fluid flow, or a concurrent fluid flow, as described in U.S. Pat. No. 4,719,173. One way to obtain this counter current or concurrent fluid flow is through the use of overflow tanks of the type disclosed in U.S. Pat. No. 4,641,941.
Regardless of the teachings of U.S. Pat. No. 4,719,173 and whether counter current or concurrent flow is employed, a substantial amount of wash water is still used because in an effort to obtain complete washing of the film, an excess of water flow is provided. It is, of course, well known in the film-developing art that the existence of residual chemicals on processed film, particularly thiosulfate, eventually results in undesirable film staining or image degradation. Yet, the obvious solution of using ample water to wash the film is becoming increasingly impractical because of the need to properly dispose of water contaminated with the chemicals washed from the film.
Many methods are available to handle the wash water problem. One method used is on-line recirculating electrolysis (ORE) of the fixer. This approach, available from both film manufacturers and equipment vendors, is primarily intended to recover silver from the fixer. However, reducing silver in the fixer also reduces the amount of silver carried from the fixer into the wash water and, as such, waste wash water quality is also improved. This method suffers from relatively high initial investment, complex operation, and high silver level in the wash water during peak film processing periods.
A lower cost option in terms of initial investment is a simple wash water recirculator that does nothing to treat the wash water. This type of simple circulator results in serious film quality problems even after only small quantities of film have been processed.
Another option available from both film manufacturers and equipment vendors is a wash water recirculating unit that treats the recirculating water and reuses all or a large fraction of the wash water. However, these units use ion exchange technology. Such units are usually effective, but they do require regular, routine maintenance or they will become ineffective. This maintenance is not only inconvenient for the user, but adds additional operating costs per unit. Since one unit is needed for each processor, annual operating costs can be quite high for a large shop.
Other technologies have been proposed and tested for treating wash water, but these have all proved either too expensive or too unreliable technically. In this category are evaporation, ozone treatment, iodine treatment, and potassium iodide precipitation.
Thus, there is a strong need to minimize the amount of wash water used in silver halide film processors, and for a method or apparatus to avoid using any excess water over what is necessary to produce complete washing of the film and at the same time minimize the amount of waste water discharged into public sewer systems.